This invention relates to glass-melting furnaces and, more particularly, to an apparatus for determining the level of batch in such furnace.
Electric glass-melting furnaces have electrodes that project into the furnace tank in a predetermined pattern, and such electrodes are immersed in the molten glass therein. Glass batch is continually supplied on top of the molten glass to provide both a source of supply and an insulating layer or crust. The glass batch is supplied by a feeder which is supported in a predetermined plane above the glass batch. The feeder is movably supported on rails located on one or both sides of the tank with the feeder being moved back and forth transversely across the furnace and longitudinally between the ends thereof to traverse the entire batch layer and to supply batch on the layer in a predetermined pattern. The batch supplied to the layer must be carefully controlled to ensure that a minimum thickness will be maintained over the entire top of the tank in order to reduce heat loss and also to protect the feeder itself against excessive heat.
In addition, it is also important that the level of the molten glass in the forehearth of the furnace which does not have any batch, be maintained constant for many operations. For example, where the glass from the forehearth is supplied to fiber-forming bushings, a change in the level of the glass in the forehearth can affect the operation of the bushings, and a fluctuation in the level can seriously hamper proper bushing throughput.
In my co-pending application, Ser. No. 864,429, filed Dec. 27, 1977, now U.S. Pat. No. 4,194,077, issued Mar. 18, 1980, there is disclosed a sensing means for determining the level of batch in a glass-melting furnace without physically contacting the batch. While this apparatus gives highly satisfactory results and represents a marked improvement over the sensing devices previously known in the art, I have now discovered that even more accurate sensing can be achieved by utilizing means for disturbing thermal inversion layers in the gaseous medium between the sensor and the top of the batch.
Ultrasonic ranging in gaseous media, as disclosed in my co-pending application Ser. No. 864,429, operates by measuring the time required for an acoustic wave emitted from a transmitter to travel from the transmitter to the top of the batch and back to a receiver after being reflected from the surface of the batch. A thermal inversion layer, i.e., an interface between cold and hot gas layers caused by a reversal of the normal temperature gradient, between the transmitter and the batch may also cause reflections. Such reflections result in an inaccurate measurement of the level of the batch, and, consequently, an incorrect adjustment in the amount of batch provided by the batch feeder. In certain instances, the error created can be quite large. For example, in one type of furnace the thermal inversion layer generally occurs at a distance of 3 to 5 inches above the surface of the batch with the normal distance between the sensor and the surface of the batch being in the range of 71/2 to 161/2 inches.
Therefore, it is an object of this invention to provide a reliable and accurate ultrasonic sensing apparatus for determining the level of the batch in a glass melting furnace without being affected by thermal inversion layers in the gaseous medium of the furnace.